A typed binding. Appears in dependent function spaces, typed lambdas, and
telescopes. I might be tempting to simplify this to only bind a single
name at a time. This would mean that we would have to typecheck the type
several times (x,y:A vs. x:A; y:A). In most cases this wouldn't
really be a problem, but it's good principle to not do extra work unless
you have to.